What is the reflection system?

How is reflection used to survey the seabed structure?

The principle of the reflection method is similar to the way you choose a tasty-looking watermelon: by giving it gentle taps. The multichannel and single-channel reflection methods provide a system by which you can identify the internal structure of the Earth according to this principle of tapping an object to understand what is inside. Entrusted with the Japan Agency for Marine-Earth Science and Technology's reflection survey methods, Nippon Marine Enterprises supplies the agency with data that it uses for investigating and understanding how earthquakes occur in the Japanese archipelago. The following shows the principle of the reflection method and the equipment that is required.

How the seismic reflection survey system works

The instantaneous release of compressed air with an air gun generates strong sound waves that will be returned after being reflected from the seafloor surface or a geologic stratum surface. The sound is received by a streamer cable in order to observe the behavior of the geologic strata. The streamer cable contains a hydrophone, which is similar to the microphone of a karaoke machine. For a one-channel geophone, each time a sound is oscillated by an air gun, a piece of signal data (see the figure above) is recorded. Repeating the cycle of air-gun oscillation and signal recording as the ship advances at a speed of about 4 to 6 knots creates the data for a strata cross-section diagram, as shown in the figure below. Two different systems are available: a single-channel system (SCS), which employs a streamer cable with only a single receiving channel, and a multichannel system (MCS), which employs a streamer cable with several receiving channels. When the multichannel system is used, multiple items of data are obtained simultaneously from a single oscillation.

Example of the result of data analysis from a seismic reflection survey

Equipment used in a seismic reflection survey

Air gun

Compresses air at high pressure and then releases the air instantly to produce extremely strong sound waves.

LL gun

Pressure of 140 atm
Eight guns with an air capacity of 24 liters

GI gun

Pressure of 140 atm
Air capacity
1.72 liters of G plus 0.74 liters of I
1.72 liters of G plus 4 liters of I

A structure with right and left wings that is equipped with a depth sensor and a magnetic compass. The purpose of a bird is to maintain the depth of the streamer cable at a constant value and to identify the degree of bending exhibited by the cable.

As an example, a 168-channel streamer uses 17 birds.

Tail buoy

Connected to the very end of a streamer cable for monitoring purposes.

Recording and gun control system

A computer system that controls the oscillation of an air gun and records the data received by a streamer cable.

Positioning device

A computer system that controls the D-GPS information. The system issues an oscillation instruction for the air gun at a suitable timing and records the required position information, such as the air-gun oscillation position and oscillation-receiving position.

On-board data processor

Processes the data that is acquired and outputs a strata cross-section diagram as the processing result. As an example of processing, since the data usually contains not only reflected waves, which is the necessary information, but also noise and other unwanted information, the processing removes the noise and unwanted data and amplifies any weak reflected signals.